Yes, the SBX game is hosted on a map called "Atlantis". You'll need to put the Atlantis .GOB file in your episode directory, then you'll need to get "SBX 3.1", create a folder called SBX in the same folder as JK.EXE, put the SBX .GOB file in the SBX folder, then run JK.EXE using -path SBX.

An alternative, is you can use the program "Patch Commander" to toggle SBX (and other mods) on and off (you'll have to launch JK from Patch Commander to play mods this way)

The other game on JKDS uses default maps with no mods, and you should be able to get into that one without changing anything.

irc://irc.holonet.org/jk - Harsh, anti-SBX community, but you'll get more games, with more players, on faster paced maps. (you'll need mIRC to connect)